The invention relates to a process for delignification of a chemical paper pulp.
It is known to apply to unbleached chemical paper pulps obtained by cooking cellulosic matter in the presence of chemical reactants a sequence of delignifying and bleaching treatment stages involving the use of oxidising chemical products. The objective of the first stage of a conventional sequence of bleaching chemical pulp is to complete the delignification of the unbleached pulp, such as it is after the cooking operation. This first delignifying stage is conventionally carried out by treating the unbleached pulp with chlorine in an acidic medium or with a chlorine-chlorine dioxide combination, as a mixture or sequentially, so as to cause a reaction with the residual lignin in the pulp and to give rise to chlorolignins which will be capable of being extracted from the pulp by solubilisation of these chlorolignins in an alkaline medium in a subsequent stage of treatment.
For various reasons it is found to be useful, in some situations, to be able to replace this first delignifying stage with a treatment which no longer calls for a chlorine-containing reactant.
It has already been proposed to treat a kraft pulp using a first stage with oxygen, followed by a stage with peracetic acid at 70.degree. C. in the presence of diethylenetriaminepentaacetic acid (DTPA) (Patent Application JP-55/94811 in the name of Mitsubishi Gas Chemical). In this known process the presence of the DTPA stabiliser prevents a considerable degradation of the cellulose chains. However, the protective effect of the stabiliser does not yet reach the sufficient level required for the production of high quality pulps.